This invention relates to manufacturing of workpieces and more particularly to apparatus and methods for positioning and holding workpieces during manufacturing operations.
In the manufacture of formed sheets of composite materials, for example, the material sheet or workpiece is typically formed to a particular shape and then trimmed to precisely fit the particular application. In the case of composite materials as used in the manufacture of aircraft, although the material is preformed to an appropriate shape, the material will bow or flex when removed from the tool used in forming. Therefore, when trimming the material, it is desirable to hold the material in its designed shape to allow precise trimming. In the past, the material would be held through the use of dies or tools which were of the same shape as the tool employed during forming. The material would be placed against the tool and held via clamping while further manufacturing operations were performed. Such operations may have included, for example, trimming the material to a proper size and shape. In accordance with this prior method, because each material sheet would likely have a contour that depended on the particular application, a separate tool was required for each part. The associated costs in storing and the setup time required to employ these tools was high. For example, in manufacturing a stabilizer from composite materials for a jet airliner, the tool necessary to hold the composite material in shape during trimming operations would be massive and of great weight, just in order to be able to support itself. The tool would then need to be stored when not in use and methods of moving the tool around to bring it to the appropriate point in the manufacturing area would need to be devised.
In order to avoid the tooling problem noted hereinabove, variable tooling fixtures have been devised wherein the shape or contour of the tooling fixture may be changed to accommodate a particular application. However, in order to have precise shapes required for aerodynamic surfaces, for example, it is desirable to have a workpiece precisely held to a tooling fixture. In the past, clamps have been used to hold material sheets to these fixtures, but such methods do not sufficiently hold the workpiece to the tool in some applications. Further, it often necessary to remove the clamps to enable trimming around a clamped area, slowing down operations and requiring manual intervention to reposition the clamps, thereby making the manufacturing process less automated. To avoid the clamping problems, systems employing vacuum suction to hold the workpiece to its particular shape have been devised. In such systems, it becomes desirable to determine whether the vacuum contact has been made as desired and to quickly locate any points on the workpiece where the workpiece is not pulled to the tool as tightly as needed for precise operations.
When devising a variable shape tooling fixture, it is desirable to be able to accurately determine the points in space where the tooling fixture contacts a particular workpiece, in order to be able to precisely hold the workpiece to a known position in space. A factor that complicates making such a determination is that the material of which the workpiece is comprised may be susceptible to damage if a tooling fixture employs a point contact with the workpiece. A surface area of contact must be used in order to avoid damaging the workpiece. However, if the tooling fixture employs a surface to contact the workpiece, determining the precise point in space of the workpiece when juxtaposed with the tooling fixture becomes difficult, since in a general case, the workpiece will be curved rather than flat. The tooling fixture must therefore provide a non-point contact with the workpiece but still be able to adapt to a general curved surface, while allowing precise determination of the position in space where the tooling fixture and workpiece make contact. Systems employing a vacuum have in the past used articulated vacuum cups that would tilt in two planes. However, providing vacuum through the area of articulation results in a complicated system which is difficult to manufacture in order to supply the needed vacuum while still allowing articulation.
Another problem arises when cutting workpieces of composite materials, for example, when the composite material includes graphite. Trimming operations in such case are likely to generate graphite particles which are conductive and therefore harmful to electrical systems unless precautions are taken to prevent graphite dust from reaching electrical systems. Therefore, any tooling fixture must be protected against damage from such particles.